For recent electrophotographic photoreceptors, durability is more required than before in view of both electrical properties and mechanical properties. Of these, in view of mechanical properties, in order to cope with long-term use, it is one problem to improve abrasion resistance of the outermost surface of the photoreceptors. As technologies for solving the problem on the abrasion resistance, there have been disclosed a technology of forming a surface layer on the outer surface of the photosensitive layer (Patent Documents 1 and 2), a technology of adding an inorganic compound to the photosensitive layer (Patent Documents 3 and 4), use of a novel photosensitive layer (Patent Documents 5 and 6), and the like.
Of these, the addition of an inorganic compound is the most easily applicable technology but the effect is not exhibited unless a certain degree of amount of the compound is added. On the other hand, when it is added in a large amount, it becomes a problem to maintain a dispersion state of inorganic particles in a coating liquid for the photosensitive layer mainly composed of organic compounds.
In the case where the dispersion state of the inorganic particles is not homogeneous, aggregation of the inorganic particles occurs on the surface of the coating liquid for the photosensitive layer to cause image defects such as density unevenness and colored spots. As a mean for solving such a problem on particle dispersion stability, there is known a method of incorporating an additive, for example, a dispersing agent such as a polyester resin or an acrylic resin (Patent Document 7).
On the other hand, a charge transport layer of a lamination-type electrophotographic photoreceptor or a photosensitive layer of a monolayer-type photoreceptor in which the inorganic particles are incorporated contains a charge transport substance and a binder resin as main components. For selecting a charge transport substance to be used, it is necessary to know an information that what kind of a series of process designs of charging, exposure, discharging, and the like are performed in an objective copier or printer, as a basic information and, based on the information, the charge transport substance is selected in consideration of properties derived from the molecule or electrical characteristic knowledge such as charge transporting ability or residual potential of the charge transport substance.
In recent years, charge transport substances having a triphenylamine structure have been frequently used. Of these, monotriphenylamine derivatives are simple in the synthetic route and are relatively easy to obtain raw materials thereof and easy to perform molecular modification, so that many compounds have been proposed and used (Patent Documents 8 and 9).